There are extensive biological, medical and industrial uses for small polymeric particles having uniform size and even more extensive uses for magnetic polymeric microspheres. Small polymeric microspheres, especially those containing covalent binding functional groups, are finding increasing uses in separation processes such as affinity chromatography, in labelling and sorting of biological cells, in diagnostic testing and in clinical treatment. Metal and metal oxide containing microspheres, particularly those containing magnetically susceptible materials, find use in catalysis and electron microscopy. Uniformly-sized particles can be utilized to calibrate instruments or filters and the like.
Magnetic particles also find use in biology as substrates or carriers for enzymes or proteins and in cell biology as substrates derivatized with ligands capable of labelling specific cells. The labelled cells can then be separated from a mixture containing both labelled and unlabelled cells or from mixtures of labelled cells with other proteinaceous material. Magnetic microspheres can also be utilized to deliver a pharmaceutical to a specified location or organ in an animal or person.
Large, uniform, biocompatible particles are needed for chromatography, affinity chromatography and cell separation procedures.
A chromatography column is made by packing a slurry of small particles into a tube to form a filter bed. Channels for liquid flow form between the particles. Columns are widely used to separate chemical mixtures, proteins or cell populations. Cells or proteins with different affinities for the packing are more or less retarded in their flow and become separated into bands as they flow through the column. If very similar compounds can be separated, the column has high resolution. When packing particles differ in size, smaller particles can clog many of the larger channels and lower flow rate. With a distribution of channel sizes, parts of the mixture flow faster through larger channels into adjacent bands, lowering the column's resolution. Uniform particles forming homogeneous packings give the highest resolution and flow rates because channel size and flow rate are everywhere the same. Hence the quest for uniform particles.
The higher the surface area, the shorter the column necessary to effect a separation. Therefore, particles near 10 .mu.m diameter are typically used for chemical separations. Small particles give slower flow rates, higher pressures and the column may clog, but this is justified for analytic preparations. For industrial scale separations, larger particles (50-100 .mu. dia.) are less troublesome. For separating cells, uniform, biocompatible particles, 100-300 .mu. dia. form channels large enough that cells can flow freely. Otherwise, cells can become trapped or damaged.
Research and medical procedures often need pure cell populations. Cells are usually too similar to be separated by simple physical properties, like density, but their surface chemistry is often unique. Monoclonal antibodies can be made that bind only to a single cell type, but we have few ways of using them for cell separations. Fluorescence activated cell sorters (FACS) are slow, have fairly low accuracy, and a high cost; panning is inefficient; and harvesting viable cells from antibody-coated netting, strings or tubes is difficult. An affinity chromatography column, with cell-specific antibodies conjugated to its packing, can separate cells. Ideally, it should have no affinity for any other kind of cell. Such non-specific binding is less important in cell depletions, where cells to be removed are adsorbed on the packing. In positive selection, however, desired cells are bound to the packing. Any non-specific adsorption lessens the accuracy of separation. Biocompatible materials with low non-specific binding are needed.